This invention relates to a method of making a water laid fibrous web such as a paper web or a web of plastics material and reinforcing fibres for consolidation or moulding into a fibre reinforced plastics sheet or article.
In paper webs, it is frequently necessary to incorporate particulate materials or powders such as pigments and fillers. In the case of webs of plastics material and reinforcing fibres one or more kinds of particulate plastics material may be included, together with materials such as pigments, fillers and antioxidants in the form of powders of substantially smaller particle size than the plastics material.
The process for making such water laid webs requires as a prerequisite the formation of an aqueous dispersion of the fibres and particulate materials from which the web is to be formed. Preferably, a foamed dispersion is used as described in the Applicants' co-pending European Application No. 85300031.3 (European Patent Publication No. 0 148 760), the subject matter of that Application being incorporated by reference herein. The dispersion so formed is then drained on a foraminous support such as the Fourdrinier wire of a paper machine, to form the web.
Two problems arise in the mechanism of more than one particulate material in an aqueous or foamed dispersion as referred to above.
First, the electrochemical conditions within such dispersions make it difficult to achieve a homogeneous mixture of the various components within the dispersion, and this is reflected as a lack of homogeneity in the web as laid down on the foraminous support.
Secondly, there will be a tendency for the particulate material to be lost during the wet laying process depending on the relative dimensions of the powder particles and the apertures in the foraminous element, for example the mesh size of a Fourdrinier wire.
When certain particles or fibres are dispersed in water, it is thought that an aqueous film forms around each individual particle or fibre and sets up an electro-chemical regime such that other particles are repelled. As a result, when fine powders are added individually, they do not agglomerate either with themselves or with other solid components of the dispersion. Thus, when the dispersion is laid down on the Fourdrinier wire, the fine particles pass through the wire with the water as drainage.